Index: src/wp-admin/includes/schema.php
===================================================================
--- src/wp-admin/includes/schema.php	(revision 26970)
+++ src/wp-admin/includes/schema.php	(working copy)
@@ -98,7 +98,7 @@
   comment_parent bigint(20) unsigned NOT NULL default '0',
   user_id bigint(20) unsigned NOT NULL default '0',
   PRIMARY KEY  (comment_ID),
-  KEY comment_post_ID (comment_post_ID),
+  KEY comment_post_ID_content (comment_post_ID, comment_content(255)),
   KEY comment_approved_date_gmt (comment_approved,comment_date_gmt),
   KEY comment_date_gmt (comment_date_gmt),
   KEY comment_parent (comment_parent)
Index: src/wp-admin/includes/upgrade.php
===================================================================
--- src/wp-admin/includes/upgrade.php	(revision 26970)
+++ src/wp-admin/includes/upgrade.php	(working copy)
@@ -411,6 +411,9 @@
 	if ( $wp_current_db_version < 26691 )
 		upgrade_380();
 
+	if ( $wp_current_db_version < 26692 )
+		upgrade_390();
+
 	maybe_disable_link_manager();
 
 	maybe_disable_automattic_widgets();
@@ -1251,7 +1254,20 @@
 		deactivate_plugins( array( 'mp6/mp6.php' ), true );
 	}
 }
+
 /**
+ * Execute changes made in WordPress 3.9.0.
+ *
+ * @since 3.9.0
+ */
+function upgrade_390() {
+	global $wp_current_db_version, $wpdb;
+	if ( $wp_current_db_version < 26692 ) {
+		drop_index( $wpdb->comments, 'comment_post_ID' );
+	}
+}
+
+/**
  * Execute network level changes
  *
  * @since 3.0.0
Index: src/wp-includes/version.php
===================================================================
--- src/wp-includes/version.php	(revision 26970)
+++ src/wp-includes/version.php	(working copy)
@@ -11,7 +11,7 @@
  *
  * @global int $wp_db_version
  */
-$wp_db_version = 26691;
+$wp_db_version = 26692;
 
 /**
  * Holds the TinyMCE version